Solid–Liquid Equilibria and Thermodynamic Modeling of a Schiff Base Ligand and Its Titanium(IV) and Zirconium(IV) Complexes

The dissolution behavior of Schiff base ligand LH₂(1) and its Ti(IV) and Zr(IV) complexes L₂Ti(2) and L₂Zr(3) was systematically investigated in five pure solvents (tetrahydrofuran, acetone, acetonitrile, ethyl acetate, toluene) from 278.15 to 308.15 K. All compounds showed positive temperature-dependent solubility. The key novelty is revealing that despite structural analogy, Ti(IV) and Zr(IV) complexes exhibit fundamentally different solvation mechanisms driven by ionic radius disparity (Ti⁴⁺: 0.605 Å; Zr⁴⁺: 0.72 Å). A dual-mechanism model is proposed: for L₂Ti(2), the smaller Ti⁴⁺ creates a congested coordination sphere, rendering solubility sterically controlled with optimal performance in acetone; for L₂Zr(3), the larger Zr⁴⁺ establishes an open coordination environment, shifting the mechanism to coordination strength control with tetrahydrofuran as the preferred solvent. These mechanisms are corroborated by Hansen solubility parameters, Gutmann donor numbers, molecular electrostatic potential analysis, and Hirshfeld surface analysis, the latter revealing reduced H···H contacts (41.5%) for L₂Ti(2) and diagnostic Zr···O contacts (0.6%) for L₂Zr(3). Seven thermodynamic models were applied, with the Wilson model best fitting LH₂(1) (ARD = 0.722%), while polynomial and Yaws models performed best for the complexes, reflecting their more complex dissolution behavior. This work provides a predictive basis for rational solvent selection in catalytic and crystallization processes, bridging fundamental coordination chemistry with industrial applications.